I can see daylight shining through where my forged upper and lower receivers meet, and it's more accurate than I am.

I shouldn't say that it absolutely will not have any effect on accuracy, as I can see how a very poor fit might. As long as the receivers are made by reputable manufacturers then you will have zero accuracy issues caused by the fit between the upper and lower.

Mine is a Palmetto State barrel and upper on a Bravo Company blem lower.